Wafer scanners are used to expose photo resists formed on wafers. The optical performance of a wafer scanner may be improved when a thin water layer is present between the lens of the wafer scanner and the wafer. In the scanning process, water is supplied to, and extracted from, below the lens. Due to high speed scanning movements of the lens, instabilities in the interface between the water and the air that presses on the water occur. This may cause water loss in the form of small droplets at the receding side of the interface.
The water droplets cause the particles of the photo resist, which particles enter into the water when the water is in contact with the photo resist, to be deposited on the receding sides of the immersion hood. Such contamination may cause defects and yield degradation. Therefore, a significant amount of maintenance time is needed to clean the immersion hood.